1. Field of the Invention
The present invention relates to a method for cleaning a photomask-related substrate selected from a substrate for a photomask, a photomask blank, a photomask, and a production intermediate thereof, and, more particularly to removal of a sulfate ion attached to the surface of a photomask-related substrate and inhibition of the generation of a particle.
2. Description of the Related Art
A photomask used for wide applications including production of a semiconductor integrated circuit such as an IC, an LSI, or a VLSI, and other is basically obtained by processing a light shielding film of a photomask blank having the light shielding film formed of a thin film containing metal or a metal compound, the light shielding film formed on a translucent substrate (a substrate for a photomask), into a predetermined light shielding film pattern by using electron-beam photolithography or the like.
In recent years, patterns have rapidly become finer and finer to meet the market demands for higher-density semiconductor integrated circuits and the like. Against this background, measures are taken by planning the matter that the exposure wavelength is shortened and the numerical aperture of a lens is increased to increase the resist resolution in an exposure process.
In photolithography method used for production or the like of the semiconductor integrated circuit described above, as an original drawing used for printing a circuit pattern on a photoresist, a photomask having a light shielding part made of a metal compound, the light shielding part formed on a substrate which is transparent with respect to exposure light is used.
However, as the photomask has come to be used for exposure of an extremely fine pattern, even extremely fine foreign matter and haze (cloudiness) become a defect. Therefore, the photomask and a material for producing the photomask are required to be extremely clean.
The photomask such as a binary mask having a light shielding part that blocks exposure light almost completely and a halftone phase-shift mask that prevents a reduction in contrast caused by diffraction of the exposure light by inverting the phase of light with respect to a light transmission part while attenuating the light has become commercially practical. These photomasks have a light shielding part made of a chromium compound or a metal silicide compound on a transparent substrate made of quartz, CaF2, or the like.
Moreover, the photomask is produced by forming a thin film made of the above-described light shielding film material on the transparent substrate described above, forming a resist pattern thereon by electron-beam lithography or the like, and transferring the pattern to a light shielding material by etching. As described above, the photomask is required to have an extremely high degree of cleanliness, precise cleaning is performed in each process.
However, the use of a high-energy beam such as ArF excimer laser light as exposure light causes a phenomenon in which a sulfate ion and an ammonium ion remaining on the substrate form an ammonium sulfate microcrystal, and this microcrystal becomes a particle and is counted as a defect. Such a phenomenon has become a problem (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2005-202135). Although the method by which a photomask material is rinsed with heated pure water has been proposed earlier in Japanese Unexamined Patent Publication (Kokai) No. 2004-19994 by the applicant of the present invention, it has been shown in Japanese Unexamined Patent Publication (Kokai) No. 2004-53817 that this method in which hot water is used is more effective in removing a sulfate ion as compared to a case in which cleaning is performed by using room-temperature water.
However, even when cleaning is performed by using heated pure water as described above, a particle is generated after drying the substrate.
Moreover, cleaning of a substrate made of quartz, CaF2, or the like, for producing a photomask blank or cleaning of a photomask blank is performed as follows. After cleaning using a surface-active agent is performed, multistage cleaning using hydrogen water or ozone water is performed, and rinsing using ultrapure water is performed in each stage as needed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2001-96241 and Japanese Unexamined Patent Publication (Kokai) No. 2002-151453).
On the other hand, even when the above-described multistage cleaning is performed, the obtained substrate is not always unconditionally clean, and fine foreign matter is generated at significant frequency. For example, when a drying method is not appropriate, contamination (foreign matter) sometimes occurs when removing a water droplet by drying (see Japanese Unexamined Patent Publication (Kokai) No. 2004-19993).